The present invention relates to fastener inserts and, more particularly, to anti-galling inserts formed from a hardened stainless steel metal alloy which are useful in both substrates and nut assemblies.
Fastener assemblies come in a variety of shapes, sizes, designs and materials. Many fastening assemblies include not only a fastener such as a bolt, pin or screw, but also will include a fastener insert to be positioned within a tapped hole. One specific type of fastener insert useful in association with a threaded fastener is the helically coiled wire insert as described in U.S. Pat. No. 2,672,070 entitled xe2x80x9cWire Coil Screw Thread Insert for Molded Materialxe2x80x9d, for example.
Generally speaking, tapped threads are strengthened due to the inherent flexibility of such helically coiled wire inserts since the insert provides a more balanced distribution of dynamic and static loads throughout the length of thread engagement. This flexibility also compensates for variation in lead and angle error.
Additionally, since the helically coiled insert does not exhibit staking, locking or swaging and does not require keying in place, helically coiled wire inserts greatly reduce stress which would otherwise be transferred to the receiving substrate. While such helically coiled wire inserts are generally useful as anchoring mechanisms for threaded fasteners in order to be used in high strength applications, such inserts must be formed from high strength materials. Heretofore, 302/304 stainless steels have been used to manufacture fastener inserts.
Due to the intimate connection between the threads of the fastener and the insert, shearing forces are exerted on the fastener insert upon attachment of the fastener. As a result of the shearing forces, fastener inserts formed from 302/304 type stainless steels have exhibited a propensity to galling despite such stainless steels being considered among the hardest in the industry.
Although galling and wear may occur under similar conditions, the types of deterioration involved are not similar. Galling occurs as a result of movements occurring along a metal-to-metal contact in a localized area which results in grooving and self-welding of the metals at the localized areas. This, in turn, leads to seizure of the metal parts.
On the other hand, wear is synonymous with abrasion and can result from metal-to-metal contact or metal-to-nonmetal contact. Such wear is characterized by relatively uniform loss of metal from the surface, as contrasted to localized grooving with consequent metal build-up, as a result of rubbing a much harder metallic surface against a softer metallic surface.
An article by Harry Tanczyn, entitled xe2x80x9cStainless Steel Galling Characteristics Checkedxe2x80x9d in STEEL, Apr. 20, 1954 points out that stainless steel sections at a relatively high hardness level or with a substantial difference in hardness, exhibit better resistance to galling than the combination of two soft members. This may be explained by the theory that the hardened sections deform elastically near the contact points under loading, while the softer pieces yield plastically for a significant distance beneath the contact points. During movement, the hardened surfaces apparently recover elastically with decrease in pressure, and this motion tends to sever any metallic welding.
Among the numerous prior art steels currently available, the austenitic Type 304 is suited to a variety of uses involving welding and fabrication, but the galling and wear resistance of this steel are poor and the metal is likely to fail when subjected to such conditions.
In view of the perceived problems with galling, the art has recently turned to cobalt bearing and high nickel alloys which are known to fight wear and galling.
It is therefore a primary object of the present invention to provide stainless steel fastener inserts which tend to limit, if not eliminate, galling in fastener assemblies.
To accomplish this objective, among others, the present invention relates to an anti-galling fastener insert comprising a body formed from a nitrogen strengthened stainless steel alloy. The stainless steel alloy preferably will include a positive amount up to a total of about 0.8% nitrogen. More particularly, the present invention relates to a nitrogen strengthened stainless steel insert formed from an alloy comprising: a) from about 0.05 to 0.15% carbon; b) from about 5.0 to 12.0% manganese; c) from about 2.0 to 6.0% silicon; d) from about 12.0 to 20.0% chromium; e) from about 6.0 to 12.0% nickel; f) from about 0.02 to 0.8% nitrogen; with the remainder being iron.
Still more preferably, the fastener insert is formed from a nitrogen strengthened stainless steel alloy comprising a) from about 0.07 to 0.12% carbon; b) from about 7.0 to 10.0% manganese; c) from about 3.0 to 5.0% silicon; d) from about 14.0 to 18.0% chromium; e) from about 7.0 to 10.0% nickel; f) from about 0.06 to 0.3% nitrogen; with the remainder being iron.
According to a highly preferred embodiment, the fastener insert will be formed from a nitrogen strengthened austenitic stainless steel alloy comprising: a) from about 0.08 to 0.1% carbon; b) from about 7.0 to 9.0% manganese; c) from about 3.5 to 4.5% silicon; d) from about 16.0 to 18.0% chromium; e) from about 8.0 to 9.0% nickel; f) from about 0.08 to 0.18% nitrogen; with the remainder being iron.
After forming the alloy into a round wire and allowing the wire to cool to room temperature, the wire is shaped to a diamond cross section and subsequently formed into insert. The resulting fastener inserts should have excellent anti-galling characteristics at both ambient and elevated temperatures. Further, the fastener inserts should have good corrosion resistance and a room temperature yield strength which surprisingly is almost twice that of fastener inserts formed from 304 type stainless steel. The nitrogen strengthened stainless steel fastener inserts of the present invention also provide excellent oxidation resistance and excellent impact strength, particularly at sub-zero temperatures.
The present invention also relates to an anti-galling free running nut assembly. The nut assembly includes a nut having an internal screw thread barrel and a helically coiled fastener insert formed from an alloy comprising: a) from about 0.05 to 0.15% carbon; b) from about 5.0 to 12.0% manganese; c) from about 2.0 to 6.0% silicon; d) from about 12.0 to 20.0% chromium; e) from about 6.0 to 12.0% nickel; f) from about 0.02 to 0.8% nitrogen; with the remainder being iron.